Achieving reliable high data rate communications over long range underwater acoustic channels is achievable only if one can detect and rapidly account for the very rapid changes in the underwater acoustic channel. In the Past acoustic communication has only been possible with slower incoherently demodulated frequency shift keying (FSK). While coherent PSK systems have been used to transmit data, they have only been used in a vertical direction in deep water applications where no multipath or channel fluctuations occur.
Assuming the harsh marine environment can be overcome, a high data rate digital underwater communications system has instant application to military surveillance, pollution monitoring, production oil field control, command and monitoring, remote sensing and even scuba diver communications. To date such applications are limited primarily due to signal fading and multipath problems as well as turbulence and Doppler induced artifacts, which severely limit realization of any practical system.
To understand the difficulties of underwater communication, it will be appreciated that the underwater acoustic channel is characterized as a time dispersive, rapidly fading channel, which in addition exhibits Doppler effects. Also, a major obstacle for satisfactory synchronization is the time varying intersymbol interference (ISI) and Doppler shifts associated with moving vehicles.
As will be appreciated, underwater acoustic data telemetry is thus a taxing problem because many unique channel characteristics such as fading, extended multipath, and refractive properties of the sound channel preclude direct applications of classical communication techniques. Over the years there have been many attempts to design a reliable acoustic link, largely by grafting methods developed for other channels. Many of these approaches fail in the aquatic environment. Of particular interest is the failure of frequency shift keying to provide bandwidth efficiency for high data rates.
The following describes both the harsh environment for underwater acoustic communications and several prior techniques attempting to combat the problems associated with the harsh environment.